1. Field of the Invention
The present invention relates to thin film transistors and methods for manufacturing the thin film transistors. The present invention further relates to display devices and electronic devices which include the thin film transistors and to which the methods for manufacturing the thin film transistors can be applied.
2. Description of the Related Art
In recent years, thin film transistors (TFTs) each including a thin semiconductor film (with a thickness of greater than or equal to approximately several nanometers and less than or equal to several hundreds of nanometers) over a substrate having an insulating surface (e.g., a glass substrate) have been attracting attention. The development of TFTs, for example, as switching elements of a display device such as a liquid crystal display device and the like has been accelerated. For the TFTs, an amorphous semiconductor or a polycrystalline semiconductor is mainly used. Further, TFTs in which a microcrystalline semiconductor is used are also known (e.g., Patent Document 1). In a display device, the switching characteristic of the mounted TFTs has influence on a display quality and power consumption.
One of the parameters that determine the switching characteristic of the TFT is an on/off ratio of current. In order to increase the on/off ratio of current, on-state current may be increased and off-state current may be decreased.
Note that “on/off ratio” in this specification means a ratio of the on-state current to the off-state current of a TFT. The “off-state current” means a current flowing between a source and a drain when the TFT is in an off-state while the “on-state current” means a current flowing between the source and the drain when the TFT is in an on-state.